This is one of those "grey" areas
If you dig down into the nuts and bolts of distance selling regulations you may well find that the company is obliged to pay return postage
But if you read the terms and conditions of the seller's web site, you'll often find this is confined to a case where the merchandise is "faulty"
This may, or may not, contravene the regulations, but...
I long ago came to the conclusion that there was more to life than starting a mega discussion with the supplier about this and, on the few occasions I've been troubled by such a happening I just send the thing back for a refund and pay the postage. Job done, move on.
However, with folk like Amazon, where I have probably bought many £100's of stuff over the last year, they just don't create any difficulty because they know I'm a good customer and don't complain without reason